Ceramic honeycomb structures of the type used in vehicular exhaust systems are known in the prior art. Such structures generally comprise a network of interconnected walls (webs) that form a matrix of elongated, gas-conducting cells which are typically square or hexagonal in shape. The cell matrix is surrounded by a cylindrical outer skin to form a can-shaped structure having opposing inlet and outlet ends for receiving and expelling exhaust gases through the matrix of cells. Such ceramic honeycomb structures find particular application as catalyst-supporting substrates for automobile exhaust systems.
When such ceramic honeycomb structures are used as automotive catalytic converters, the cell walls are coated with a precious metal catalyst containing platinum, rhodium or palladium, for example. Such structures have a cell density of approximately 400 to 900 cells per square inch in order to maximize the area contact between the automotive exhaust gases which blow through the gas conducting cells, and the catalyst present on the walls. To reduce the pressure drop that the exhaust gases experience when flowing through the honeycomb structure, the walls are typically manufactured between 3.0 and 5.0 mils thick. The use of walls of such thickness also results in a reasonably short light off time of about 24 seconds (i.e., the time it takes before the webs reach the required 250° C. before the catalyst impregnated over the walls begins to oxidize CO to CO2, and to effectively disassociate NOx into N2 and O2). A short light off time is important, as most of the automotive pollutants generated by an automobile with a catalytic converter are produced in the time between the automobile is first started until when the walls reach the required activation temperature of about 250° C.
In order to reduce the light off time even further, ceramic substrates having very thin walls, on the order of 2 mils or less, have been manufactured. However, the applicants have observed two major shortcomings associated with such thin-walled substrates. First, such substrates are structurally weaker than more conventional, thicker walled substrates. Hence, they are more prone to crack or to break during both the manufacturing process and the “canning” process when they are locked into a metal enclosure that forms part of the automotive exhaust system. Secondly, in operation, the thin walls may be prone to erosion along the face of the inlet end of the substrate due to the impingement of the particulate material entrained in the stream of exhaust gases. The resulting “sand blast” effect may weaken the already fragile structure, but may also render the inlet portion of the substrate ineffective in catalyzing pollutants by eroding away the catalytic coating on the cell walls, or even the walls themselves. In lieu of providing thinner walls to reduce the light off time of such ceramic substrates, substrates having the same thickness but a higher porosity have also been manufactured. However, the applicants have observed that these substrates suffer from the same deficiencies as thin-walled substrates do, i.e., insufficient mechanical strength, and excessive erosion near the inlet end.
Clearly, what is needed is an improved catalytic flow-through ceramic substrate having a shorter light off time without a significant reduction in the mechanical strength of the resulting substrate. Ideally, such a substrate would also have an erosion resistance at its inlet end that was at least comparable to the erosion-resistance of prior art ceramic substrates employing web walls between 3.0 and 5.0 mils thick. It would be desirable if such a fast light off substrate further had slower cool down properties to reduce the thermal gradient between the centroid and the outer skin of the substrate that can sometimes cause cracking due to differences in thermal expansion in these regions. Finally, it would be desirable if such a fast light off substrate were relatively easy and inexpensive to manufacture.